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A090368
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a(1) = 1; for n > 1, smallest divisor > 1 of 2n-1.
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20
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1, 3, 5, 7, 3, 11, 13, 3, 17, 19, 3, 23, 5, 3, 29, 31, 3, 5, 37, 3, 41, 43, 3, 47, 7, 3, 53, 5, 3, 59, 61, 3, 5, 67, 3, 71, 73, 3, 7, 79, 3, 83, 5, 3, 89, 7, 3, 5, 97, 3, 101, 103, 3, 107, 109, 3, 113, 5, 3, 7, 11, 3, 5, 127, 3, 131, 7, 3, 137, 139, 3, 11, 5, 3, 149, 151, 3, 5, 157, 3, 7
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OFFSET
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1,2
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COMMENTS
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For n > 1, a(n) is an odd prime. - Chai Wah Wu, Nov 14 2022
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LINKS
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MATHEMATICA
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Join[{1}, Divisors[#][[2]]&/@Range[3, 161, 2]] (* Harvey P. Dale, Oct 09 2011 *)
a[n_] := FactorInteger[2*n-1][[1, 1]]; Array[a, 80] (* Amiram Eldar, Jun 11 2022 *)
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PROG
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(Python)
from sympy import primefactors
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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